Determination of creatine, creatinine, arginine, guanidinoacetic acid, guanidine, and methylguanidine in biological fluids.

The non-specific analytical methods for the guanidinium compounds in biological fluids have been unsatisfactory for many years. The methods presented in this paper have a greatly improved specificity and are not too complex for use in the clinical laboratory. A modified Sakaguchi color reaction for substituted guanidines is used in each procedure. Guanidinoacetic acid is measured by developing the color reaction with a Ba(OH)s-ZnS04 protein-free filtrate. Arginine plus guanidinoacetic acid is measured by the color reaction with a tungstic acid protein-free filtrate. Creatinine is degraded to methylguanidine for measurement. Creatine is converted to creatinine by heat and acid and then degraded to methylguanidine. Methylguanidine and guanidine are determined by developing the color reaction on a Ba(OH)z-ZnSOa proteinfree filtrate (arginine-free) which has been treated with a strong anion exchange resin to remove guanidinoacetic acid. Any guanidine present must first be treated with dimethyl sulfate before the color reaction is applied. ReagentsProtein precipitation reagents. 1. Q N H804 and 10 per cent sodium tungstate (1, 2). 2. 0.3 N Ba(OH)z (3).’